The present invention relates to the manufacture of printed circuit boards and, more particularly, to the manufacture of double-sided and multilayer printed circuit boards containing thru-holes requiring metallization.
In the manufacture of printed circuits, it is now common-place to provide planar boards having printed circuitry on each side thereof. Also gaining increased importance are so-called multilayer circuit boards comprised of integral planar laminates of insulating substrate and conductive metal (e.g., copper), wherein one or more parallel innerlayers or planes of the conductive metal, separated by insulating substrate, are present within the structure. The exposed outer sides of the laminate contain printed circuit patterns as in double-sided boards, and the inner conductive planes may themselves comprise circuit patterns.
In double-sided and multilayer printed circuit boards, it is necessary to provide interconnection between or among the various layers or sides of the board containing conductive circuitry. This is achieved by providing metallized, conductive thru-holes in the board communicating with the sides and layers requiring electrical interconnection The predominantly-employed method for providing conductive thru-holes is by electroless deposition of metal on the non-conductive surfaces of thru-holes drilled or punched through the board.
A typical manufacturing sequence for producing double-sided or multilayer printed circuit boards containing metallized thru-holes is illustrated in the flow diagram, FIG. 1, and its accompanying FIG. 2 showing in cross-section and greatly expanded scale the corresponding structure of the board at particular steps in the sequence. In the sequence shown, there is first provided a copper-clad substrate consisting of a nonconductive substrate 10, typically an epoxy glass resin, having applied to it on both sides thin copper foil laminate 12. Thru-holes 14 are drilled in the laminated board, exposing hole surfaces of nonconductive substrate material 10. The board is scrubbed and the drilled holes de-burred, followed by the variety of steps required to plate the thru-hole surfaces with conductive metal. Thus, the boards are generally racked, cleaned and conditioned, and subjected to a micro-etch process to render the copper surfaces receptive to adherence of subsequently-applied activator/catalyst; the drilled thru-holes generally are already sufficiently roughened by the drilling operation to render them receptive to catalyst adherence, although sometimes a glass etch is employed to frost exposed glass fibers, in those boards made of glass filled substrate resin, to improve plateability of the glass fibers. Although not shown in the flow diagram, cold water rinses are usually employed after each particular processing operation. The activator (catalyst) is then applied to the exposed surfaces, and the activator then accelerated as known in the art. Electroless copper 16 is then deposited on the activated surfaces resulting in metallization of the thru-hole surfaces. Additional metal build-up on the hole surfaces and at those areas which will define conductive circuitry (pads, traces, etc.) is then provided, after rinsing and drying of the board, by application of a plating resist 18 in a predetermined pattern (generally via application of a photoresist, exposure through a mask in the desired pattern and development or, alternatively, by screen printing of resist in the pattern), followed by, e.g., copper electroplating to provide additional copper 20. Desired metallized areas are then protected with an etch resist, the plating resist removed, and those areas unprotected by etch resist then etched down to the substrate surface.
There are a number of known variations on the foregoing sequence which are practiced in the art. The illustrated sequence typically is referred to as a "heavy deposition" process in which the layer of electroless copper deposited is about 80-100 millionths of an inch. In a variation, the electroless copper is deposited to a thickness of only about 15-20 millionths of an inch ("thin deposition"), and is then followed by an electrolytic copper strike layer of about 100-200 millionths of an inch for build up of metal prior to application of resist in desired pattern. In another variation, referred to as "panel plating", the thin deposition of copper is followed by a plating of electrolytic copper to the full or final thickness (e.g., 1 to 1.5 mil) before any application of resist takes place. In this variation, the patterned resist serves as an etch resist rather than a plating resist, the etch resist being applied to the fully built up areas of holes, pads, traces, etc. The unprotected areas of copper are then etched down to the substrate surface.
Irrespective of the particular variation employed, the foregoing sequences have in common the need for a number of process steps. However, in terms of the economics of actual manufacturing practice, the overall number of specific steps performed is actually of less consequence than the number of different types of steps. Thus, while plating of thru-holes via heavy or thin deposition per se involves a fair number of processing and rinsing steps, all the steps are basically wet processes performable in straightforward sequence during manufacture. More consequential from an economic point of view is the need to remove the board from this wet processing sequence, dry it, subject it to photo-imaging or screen printing, and then return it to a wet processing, plating and etching sequence. The same is true for panel plating where the electroless and electrolytic processes for build up of metal are interrupted for application of an etch resist and then returned to a wet processing etching step. These interruptions in process sequences, requiring physical transport and return of boards to and from different areas of the plant and intermediate drying sequences, can add significantly to the cost of manufacture.